Voltage-to-current converters scale an input voltage into a desired output current. The converters can be used as stand alone voltage-to-current converters but more often are part of a larger system using current mode processing. The transconductance or gain of the voltage-to-current converter is generally referred to as the voltage-to-current scaling ratio, which is set by an impedance and typically by a resistive network. In order to select a desired ratio for various input ranges, a programmable gain stage having a resistive network and switches is generally used. Some examples of amplifier and resistor network arrangements can be seen in U.S. Pat. Nos. 7,162,029 and 5,808,576.
A typical configuration of a voltage-to-current converter, though, is shown in FIG. 1. FIG. 1 shows an amplifier AMP and an R-2R type resistor network. The internal or tap nodes N1 to N5 can be connected through one of the switches S1 to S5 to the negative input of the amplifier AMP. The amplifier AMP forces the voltage Vin to the negative input and the output current I of the amplifier will then assume a magnitude based on the impedance configuration at the negative input configurable by the switches S1-S5. The R-2R resistor network is compact but imposes restraining limitations on the input range of the input voltage Vin as the input signal Vin is gained up to the output of the amplifier, and therefore limited by the output swing capability of the amplifier AMP. If, for example, switch S1 is closed (e.g., conductive) and the other switches S2 to S5 are open (disconnected), the voltage level at node N1 will be Vin. The R-2R network causes the voltage levels to be 2*Vin at node N2, 4*Vin at node N3, 8*Vin at node N4 and 16*Vin at node N5. Therefore, the input voltage range for the input voltage Vin is limited to a maximum of the supply voltage level divided by 16. This limitation is even more restraining for fully differential, such as symmetric structures where the common mode input range is strongly constrained. Furthermore, the approach is also limited due to the voltage feedback gain-bandwidth relationship.